Some of the malware that knocked down the computer network of Sony Pictures Entertainment contained Korean-language code in a sign that North Korea played a role in the attack, a news report said Tuesday.

North Korea has been suspected of involvement since last Monday's cyber-attack on Sony because the communist nation has expressed strong anger at a comedy film, "The Interview," about a plot to assassinate leader Kim Jong-un. 

Sony is scheduled to release the film this month.

The FBI has launched an investigation into the case.

On Tuesday, the Bloomberg news agency reported that some of the malware used in the attack contained Korean language code, and other aspects of the breach bear important similarities to attacks that wiped out the computers of South Korean banks and broadcasters in March 2013.

The report cited two people familiar with the investigation.

Since the attack, at least five new Sony movies have been leaked online. But "The Interview" was not among them, another indication pointing to Pyongyang's possible involvement because the North would not want its people to see the film.

When the attack happened, it left Sony employees locked out of their computers. Before the screens went dark, they displayed the phrase "Hacked By #GOP", which reportedly stands for "Guardians of Peace," as well as a message that threatened to release sensitive data stolen from Sony servers if certain demands were not met, according to media reports. 

The attackers also reportedly posted messages on several Sony Twitter accounts, personally attacking Sony Pictures CEO Michael Lynton.

But a North Korea expert in Washington said he is skeptical about North Korea's involvement.

Alexandre Mansourov, an adjunct professor at the U.S.-Korea Institute at the Johns Hopkins University School for Advanced International Studies, said that the North Koreans have never made such undisclosed demands or bullied corporate managers of a target organization with Twitter posts as in Sony's case.

"I have no definitive proof that they've not done it. I'm just saying that I've seen a few tactics which were used in this particular incident, which are not characteristic of the previous North Korean attacks," he said at an event discussing the North's cyber-capabilities.

"I think we should leave it to digital forensics to determine whether it's North Koreans or somebody else." (Yonhap)